FIG. 1 illustrates a typical audio output circuit 100. An electrostatic protection circuit 102 is used to shunt harmful external static electricity away from the audio output circuit 100. A sound signal from an audio IC or a speaker is outputted through a low-pass filter 104 and the electrostatic protection circuit 102.
However, a “pop” sound is always intermixed with the outputted sound signal at the moment the power (Vcc) is turned on or off due to the voltage spiking. Typically, an anti-pop circuit 106 is installed in the audio output circuit 100 to eliminate the “pop” sound
Figure 1 illustrates a typical anti-pop circuit 106 including two NPN type bipolar junction transistors (BJTs) 108 and 110. Before the power (Vcc) is turned on, a control signal triggers the anti-pop circuit 106 first to activate the BJT 110 for preventing the “pop” sound due to a voltage spike from mixing into the outputted voice signal. Specifically, a control signal with a low voltage level is outputted first to turn off the BJT 108 so that a high voltage level is generated to turn on the BJT 110.
Typically, a special time period to keep BJT 110 turned on is set, such as 10 ms, after which the power is considered stable and not prone to cause “pop” sounds from spiking. After the set time period is reached, the control signal is transferred from a low voltage level to a high voltage level to turn on the BJT 108 so as to generate a low voltage level to turn off the BJT 110. At this time, the anti-pop circuit 106 is turned off.
Because the control signal has an inverse polarity to that of the power (Vcc), a BJT 108 is required in a typical anti-pop circuit to act as an inverter, which requires an additional cost. Moreover, the control signal is required to cooperate with turning on and off the power (Vcc) to trigger the anti-pop circuit. Therefore, controlling the time sequence of the control signal and the power (Vcc) is very important.
Accordingly, an anti-pop circuit with simple structure and simple control is necessary.